1. Field of the Invention
This invention relates to a photographic optical system controlling apparatus used in video cameras etc., which performs lens focusing control and exposure control based on image signals obtained from an imaging element.
2. Description of the Related Art
Heretofore, there have been proposed various types of automatic focusing system used as an optical controlling means of a photographic apparatus. Among them, the most popular method, using an image signal obtained from an imaging means, is to pick up the high-frequency component in the image signal and to obtain its differential value, and then to drive a lens in a direction which allows the absolute value of the differential value to be increased. An object image obtained through the lens system is most sharply outlined when in focus. Further and being out of focus in any direction causes fuzz. Therefore, the output image signal of the video camera used to photograph this object image has a maximum level of its high-frequency component when in focus.
The image can then be brought into focus by controlling the position of the lens such that the high-frequency component of the image signal becomes maximum. Such a focusing method is called the "climbing serbo method".
FIG. 12 of the accompanying drawings shows a typical automatic focusing apparatus using the climbing serbo method. In FIG. 12, the numerals designate respectively: 2, a lens; 4, an imaging element for converting the image, formed on the imaging surface through the lens, into electrical signals; 6, a pre-amplifier for amplifying the image signal output from the imaging element 4; 8, signal processing circuit for converting the output signal from the pre-amplifier 6 into a standardized signal like an NTSC signal; 10, a band-pass-filter (hereinafter referred as "BPF") for picking up only the high-frequency component of the output signal from the pre-amplifier 6; 12, a gate circuit for selecting only the signal in the focusing detecting area from the output signals of the BPF 10 forming one screen (one field or one frame), and allowing the selected signal to pass through; 14, wave-detecting circuit for wave-detecting the output of the gate circuit 12; 16, motor driving circuit for driving the lens driving motor based on the output of the wave-detecting circuit 14; and 18, a lens driving motor for controlling the focusing operation by moving the lens position.
According to this composition, the image formed on the imaging surface of the imaging element through the lens 2 is converted into electrical signals, and is then amplified to a predetermined level by the pre-amplifier 6. The high-frequency component of the image signal varies in accordance with the lens position, namely the focusing condition of the object. Specifically, the high-frequency component increases as the lens moves closer to the in focus position, and becomes maximum at the focus point.
FIG. 13 shows the variation of the high-frequency component in the image sigal with respect to the lens position. As seen from the FIG. 13, the high-frequency component becomes maximum at the focus point, and decreases as the lens moves away from the focus point. Accordingly, it is understood that the focused state can be obtained by positioning the lens at a position rendering the maximum high-frequency component.
As another optical system controlling device of a photographic apparatus, an exposure controlling device is known. The main section of such a photographic apparatus is shown in FIG. 14.
In FIG. 14, the numerals designate respectively: 20, a lens; 22, an exposure controlling circuit for controlling the quantity of light incident from the lens 20; 24, an imaging element for converting the image formed on the imaging surface through the lens 20 into an electrical signal; 26, an amplifier for amplifying the image signal output from the imaging element 24; 28, an AGC circuit for ensuring that the output signal from the amplifier 26 is constant; 30, a signal processing circuit for converting the output from the AGC circuit into a standardized (e.g. NTSC) image signal; 32, an image signal output from the signal processing circuit 30; 34, a photometric area determining circuit for determining the photometric area; 36, a gate circuit for allowing the output signal of the amplifier 26 to pass through in accordance with the timing of the photometric area, being the output of the photometric area determining circuit 34; 38, a signal level detecting circuit for detecting the luminocity information output from the gate circuit 36; 40, an exposure detecting signal generated in the signal level detecting circuit 38; and 42, an exposure control target value set by an external device.
In operation of the apparatus shown in FIG. 14, the incident light, projecting into the lens 20 and exposure-controlled by the exposure controlling circuit 22, forms an image on the imaging element and is converted into electrical signals. The output of the imaging element 24 is amplified in the amplifier 26 and input to a AGC circuit 28. The AGC circuit 28 controls the gain of the signal to make its output level constant, which is output as an image signal 32 through a signal processing circuit 30. Meanwhile, a photometric area determining circuit 34 outputs signals corresponding to the timing of a photometric frame. The output of the amplifier 26 is input to the signal level detecting circuit 38 through a gate circuit 36 in accordance with the timing. The signal level detecting circuit 38 generates an exposure detecting signal, being information of light intensity. The exposure controlling circuit 22 controls the exposure such that the level of the exposure detecting signal 40 equals an exposure control target value 42.
Next, an emphasized photometric operation provided with a photometric frame will be described hereinafter. In general, the upper part of the background of an image usually consists of a high luminace image like the sky. Therefore, if the exposure controlling is performed in accordance with the luminance level of such a high-luminance background, the image will become a so-called backlight shot causing the main object e.g. face of a person, to become darkened. To cope with this problem, there has been performed exposure control by providing a photometric frame 44 positioned at the lower central part of the screen by a photometric area generating circuit 34, and then by performing an emphasized photometric operation in the photometric frame using image signals in the frame by a signal level detecting circuit 38, as shown in FIG. 15(a). Alternatively, another type of exposure controlling operation may be carried out by dividing the screen into a plurality of portions, and weighting the luminance information obtained from each of the divided portions, as shown in FIG. 15(b).
According to the aforementioned conventional automatic focusing device, however, a disadvantage has arisen: the focusing detecting area is fixedly determined at the center of the screen, so there has been a fear of misfocusing on other objects, which should not be focused on, and are accidentally located at the center of the screen, when the position of the object to be focused and having been at the center changes owing to the movement of the camera etc. In order to avoid such an inconvenience, Japanese Patent Laid-Open No. Sho 64-49484 or Japanese Patent Laid-Open No. Sho 64-71382 teaches the use of a variable gate circuit. However, since the area determinating operation by the variable gate circuit is carried out basically by the same method as the focusing state detecting method, if the focusing operation malfunctions, the gate area determination operation will suffer degradation in accordance therewith.
Also, according to the aforementioned conventional exposure controlling device, a disadvantage has arisen: the emphasized photometric area is fixedly determined in the screen irrespective of the weighting value. Therefore, even if the position of the main object in the screen changes due to the movement of the camera etc., the exposure control operation is carried out to be optimum for any object located in the photometric area, not for the main object.